1. Field of the Invention
The present invention generally relates to a motor-driven power steering apparatus for an automobile or a motor vehicle for assisting a driver in steering the vehicle by manipulating a steering wheel. More particularly, the invention is concerned with a motor-driven power steering apparatus which can ensure improved return performance of the steering wheel without involving degradation in the linearity of assist torque control.
2. Description of the Related Art
For a better understanding of the invention, description will first be made of a motor-driven power steering apparatus known heretofore.
FIG. 5 is a block diagram showing schematically a configuration of a motor-driven power steering apparatus known heretofore, which is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 35664/1985 (JP-A-60-35664).
Referring to the figure, the power steering apparatus is equipped with a torque sensor 1 for detecting a steering torque T of a steering wheel (not shown) and a vehicle speed sensor 2 for detecting a vehicle speed V. An output shaft of an electric motor 3 is operatively coupled to the steering wheel. The electric motor 3 is electrically connected to a DC power supply source such as an onboard battery 4 via a bridge circuit constituted by two pairs of switching elements such as switching transistors, i.e., a first pair of switching elements Q.sub.1 and Q.sub.4 and a second pair of switching elements Q.sub.2 and Q.sub.3 for allowing the motor 3 to be driven selectively in either one of forward and backward directions. Fly-wheel diodes D.sub.1 to D.sub.4 are connected across the switching elements Q.sub.1 to Q.sub.4, respectively. A resistor 5 is inserted-in a current path between the battery 4 and the bridge circuit mentioned above. A motor current detecting means 6 is provided for detecting a current I supplied to the motor 3 through the resistor 5.
The outputs of the torque sensor 1, the vehicle speed sensor 2 and the motor current detecting means 6 are supplied to a signal processing unit 7 which is adapted to control the switching elements Q.sub.1 ; Q.sub.4 or Q.sub.2 ; Q.sub.3 or the basis of the steering torque T, the vehicle speed V and the motor current I and includes a target value calculating means (not shown) for arithmetically determining or calculating a target current value I.sub.0 of the motor current I on the basis of the steering torque T and the vehicle speed V as detected, a control quantity calculating means (not shown) for calculating a control quantity for controlling the motor 3 on the basis of a deviation or difference between the detected motor current I and the target current value I.sub.0, a converting means for converting the control quantity mentioned above into PWM (Pulse Width Modulated) signal for controlling the switching elements Q.sub.1 ; Q.sub.4 or Q.sub.2 ; Q.sub.3 and a driving circuit (not shown) for driving the switching elements in accordance with duty cycles or ratios indicated by the PWM signals.
Next, description will turn to operation of the conventional power steering apparatus shown in FIG. 5.
It is assumed, by way of example, that a driver of the motor vehicle tries to rotate the steering wheel in the rightward (clockwise) direction. In that case, the signal processing unit 7 outputs the driving signal for controlling conduction of the paired switching elements Q.sub.1 and Q.sub.4 in dependence on the steering torque T and the vehicle speed V as detected by the torque sensor 1 and the vehicle speed sensor 2, respectively. At this juncture, it should be mentioned that there are provided first and second driving modes for controlling the switching elements (Q.sub.1 ; Q.sub.4 or Q.sub.2 ; Q.sub.3). In the first driving mode, one of the switching elements (Q.sub.4 or Q.sub.3) in each pair of the switching elements (Q.sub.1 ; Q.sub.4 or Q.sub.2 ; Q.sub.3) is held in the conducting (ON) state while the other one (Q.sub.1 or Q.sub.2) is controlled in dependence on the duty cycle of the PWM signal. On the other hand, in the second driving mode, both of the paired switching elements (Q.sub.1 and Q.sub.4 or Q.sub.2 and Q.sub.3) are driven in accordance with the duty ratio of the PWM signal.
It is assumed again, by way of example, that the first driving mode is validated and that the switching elements Q.sub.1 and Q.sub.4 are in charge of controlling the forward rotation of the motor 3 while the switching elements Q.sub.2 and Q.sub.3 are to control the backward rotation of the motor 3.
When the driver rotates the steering wheel in the clockwise direction (which corresponds to the forward rotation of the motor 3), the signal processing unit 7 outputs correspondingly a forward motor rotation signal. In that case, one Q.sub.4 of the paired switching elements Q.sub.1 and Q.sub.4 is so controlled as to be held constantly in the conducting state while the other one Q.sub.1 is repetitively turned on and off in accordance with the duty ratio of the PWM signal.
During a period in which the switching transistor Q.sub.1 is turned on, a DC current is supplied to the motor 3 via a current path extending from the battery 4 to the ground through the resistor 5, the switching element Q.sub.1, the motor 3 and the switching element Q.sub.4, which results in that the motor 3 is rotated in the forward direction (corresponding to the clockwise rotation of the steering wheel). In this manner, the motor 3 generates an output torque of magnitude which depends on the duty ratio of the PWM signal with which the switching element Q.sub.1 is turned on and off. The output torque of the motor 3 thus aids the driver in steering the motor vehicle by reducing correspondingly the steering torque T applied by the driver. When the steering torque T applied to the steering wheel is cleared, the steering wheel automatically returns to the neutral or center position under a self-aligning torque.
As can be seen from the above description, the switching element Q.sub.4 is held in the conducting (ON) state even when the switching element Q.sub.1 is turned off in the first driving mode. Consequently, a closed circuit is formed by the switching element Q.sub.4, the fly-wheel diode D.sub.2 and the motor 3, as indicated by arrows shown in FIG. 5. Accordingly, when the motor 3 is rotated due to external forces such as a self-aligning torque, a load torque and the like, which act to return the steered road wheels to their original neutral positions, in the state mentioned above (i.e., when the switching element Q.sub.4 is in the conducting state with the switching element Q.sub.1 being off), a current flows, as indicated by the arrows, which results in that the motor 3 generates a torque which is utterly independent of the torque control. In this conjunction, it should be noted that no means is provided for turning off the switching element Q.sub.4. Consequently, the current flowing through the motor 3 in the state mentioned above can not be controlled.
The torque generated by the motor 3 independent of the power steering control, as described above, acts as a regenerative braking force when the steering wheel returns to the center position under the self-aligning torque and thus reduces the returning speed of the steering wheel.
For solving the problem mentioned above, it is conceivable to validate the second driving mode to thereby turn on/off the switching element Q.sub.4 together with the switching element Q.sub.1 in accordance with the duty ratio of the PWM signal. In that case, the frequency of the PWM signal will necessarily increase. Consequently, under the influence of inductance of the motor 3, linearity in the relation between the duty ratio of the PWM signal and the output torque of the motor 3 is degraded whereby the control performance of the power steering apparatus is lowered.
Next, differences in the return characteristic of the steering wheel and the linearity due to difference in the output torque of the motor 3 between the first and second driving modes will be elucidated below in detail.
FIGS. 6A and 6B are waveform diagrams illustrating voltages (solid-line curve) and currents I (broken-line curve) of the motor 3 in the first and second driving modes, respectively.
As can be seen by comparing the waveforms shown in the figures, the motor current I in the first driving mode differs from that in the second driving mode. Such a difference in the motor current I can be ascribed to a difference in the on/off time constant due to a difference in ohmic resistance, for example, of the switching element Q.sub.4 between the first and second driving modes. More specifically, when inductance of the motor 3 is assumed as being constant, the time constant of the motor circuit including the resistance, the switching element Q.sub.4 and the motor 3 is in reverse proportion to the on/off resistance of the switching element Q.sub.4. Thus, the time constant assumes a large value in the first driving mode in which the switching element Q.sub.4 is constantly held in the on-state while the time constant is small in the second driving mode where the switching element Q.sub.4 is turned on and off.
For the reason mentioned above, the time constant for the regenerative brake current of the motor 3 when the switching element Q.sub.4 is off in the first driving mode is large, as is shown in FIG. 6A. This means that a long time is taken for the motor current I to attenuate, although a high linearity can be assured between the duty ratio of the SW signal and the torque generated by the motor 3. Consequently, the return characteristic of the steering wheel is degraded.
On the other hand, in the second driving mode shown in FIG. 6B, the above-mentioned time constant is small. Consequently, the motor current I changes rapidly when the switching elements Q.sub.1 and Q.sub.4 are turned off. In other words, the motor current I tends to decrease to zero immediately in response to a change of the PWM signal to the off-level. However, because of poor linearity, the control of the motor current I or output torque to a desired value becomes unstable particularly in a control region where the current 1 is large, bringing about fluctuations in the output torque as well as generation of acoustic control noise by the motor 3.
FIG. 7 is a characteristic diagram illustrating relation of the motor output torque (motor current I) to the duty ratio of the PWM signal, wherein the output torque generated when the steering wheel is rotated rightwardly or clockwise is shown in the first quadrant with the output torque generated upon leftward or counterclockwise rotation of the steering wheel being shown in the third quadrant. Arrows shown in FIG. 7 indicate the direction in which the frequency of the PWM frequency increases. It will be seen from this figure that the linearity is degraded when the driving mode is changed over from the first driving mode (a) to the second driving mode (b).
More specifically, it is apparent from FIG. 7 that the output torque characteristic is represented substantially by a linear function of the duty ratio (i.e., the output torque characteristic exhibits high linearity) in the first driving mode represented by a graph (a). On the other hand, in the second driving mode represented by graphs (b), the output torque characteristic takes a curvilinear form and degradation in the linearity becomes more remarkable as the frequency of the PWM signal increases.
Further, in the second driving mode, ripple components of the motor current I generated upon turning-on/off of the switching elements Q.sub.1 and Q.sub.4 become more remarkable when compared with the motor current in the second driving mode, resulting in generation of radio noise as well as heat generation of the switching elements Q.sub.1 to Q.sub.4 and the ripple suppressing capacitor.
As is obvious from the foregoing discussion, the power steering apparatus known heretofore in which the switching elements Q.sub.1, Q.sub.4 are controlled only in one of the first and second driving modes is disadvantageous in that the return performance of the steering wheel is poor in the first driving mode and that the linearity of the PWM control is degraded in the second driving mode.